The present invention relates to an imaging medium in the form of a self-contained imaging assembly and, more particularly, to an improved self-contained imaging assembly containing a photosensitive imaging layer or layers comprising photohardenable microcapsules encapsulating a coloring material and, outside the microcapsules, a developer material disposed between a first transparent support and a second support which may be opaque or transparent. The imaging medium or assembly can also be referred to as a recording medium, and the imaging layer can be referred to as a recording layer, since the assembly can serve both to capture an image (either the original image or an electronic copy), as does film, and also to display the image, as does a print. Consistent with this fact, the imaging assembly can form a positive image.
The photosensitive imaging layer (including microcapsules) is colored by pressure development after exposure to radiation based on image information. The microcapsules, whose mechanical strength changes (increases) when exposed to light, are ruptured by means of pressure development, whereupon the coloring material and other substances encapsulated in the microcapsules flow out (to varying amounts based on the exposure) and development occurs. The coloring material, such as a substantially colorless color former, migrates to, and reacts with, the developer material and coloring occurs, whereupon a color image is developed.
The "rupture" of the microcapsules are not an all-or-nothing event. Rather, the microcapsules exposed to light are differentially photocured to release varying amounts of color former in order to achieve tonal depth in the exposed area. The differential exposure to light proportionately increases the viscosity of the photocurable composition and thus immobilizes the color former proportionately to the desired tonal depth in the exposed area. The rupture of the microcapsules and the release of the color former is accomplished by the uniform application of pressure. Development of the photosensitive imaging layer can be accomplished, for example, by passing the imaging assembly between a pair of upper and lower nip rollers.
Photohardenable imaging systems employing microencapsulated photosensitive compositions are the subject of various patents, including U.S. Pat. Nos. 4,399,209, 4,416,966, 4,440,846, 4,766,050, 5,783,353, and 5,916,727. Image forming devices (also referred to as printers) are disclosed, for example, in U.S. Pat. No. 4,740,809, wherein exposure occurs by guiding a light from a light source for a plurality of colors across a photosensitive recording medium. U.S. Pat. No. 4,992,822 discloses an image forming device, capable of producing a plurality of colors via a polygonal mirror, for repeatedly exposing the same pixels in a photosensitive recording medium. U.S. Pat. No. 5,893,662 discloses a device for printing an image wherein the device can be incorporated into a computer bay. U.S. Pat. No. 4,648,699 describes a development technique which employs, instead of a pair of nip rollers, a point contact ball moving relative to the photosensitive recording medium.
In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated together with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Then, as mentioned above, following exposure, the imaging media in the form of a sheet can be subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers.
One of the problems in providing a self-contained imaging assembly that provides a high quality print is image stability or "keeping" which is affected by humidity sensitivity. It is known that print quality, and in particular sensitometric response to actinic radiation, can be significantly affected by sensitivity to humidity and the relative humidity of the environment. Even if the media is manufactured and packaged at a particular humidity, which is found optimum for print quality, variations after the media acclimates to a different later environment can adversely affect the sensitometic properties. This has been believed due to the materials employed in the imaging media, in particular the degree of hardening or curing of the internal phase of a microcapsule and the consequent increase in the viscosity varying with a change in humidity. As a result thereof, photographic characteristics such as speed, maximum density and fogging density are changed from the original optimum. Furthermore, a full color imaging is adversely affected.
In forming a full color image, color precursors which develop into yellow, magenta and cyan colors and photo-initiators corresponding to blue, green and red lights are encapsulated in an internal phase of the microcapsules, and the three sets of the microcapsules are mixed to prepare a full color imaging material containing a developer. The photographic characteristics of the respective microcapsules vary with a change in humidity to different degrees, resulting in muddy colors or incorrect or suboptimal colors. For example, when it is desired that a yellow color be developed, cyan and magenta capsules are cured by red and green lights, and only a yellow color former reacts with a color-developer to form an image. However, if the cyan or magenta capsules insufficiently cure due to a change in humidity, the result may be a muddy color in which cyan or magenta is blended with yellow to some extent. Such muddy colors or other sensitometric phenomena due to a change in humidity has been a significant problem.
One technique that has been used to address the humidity problem and to improve media stability resides is conditioning the layer containing the developer and microcapsules to a relative humidity (RH) of about 10 to 40% and preferably about 20%. For example, U.S. Pat. Nos. 5,916,727 and 5,783,353 disclose conditioning the layer at about 20% RH for about 2 to 12 hours or more, at ambient temperatures, and subsequently sealing the assembly at this low RH level to assure that the layer is relatively moisture-free during the normal shelf-life of the assembly.
U.S. Pat. No. 5,996,793 discloses storing the image-forming material together with a humidity-controlling material. Further, the patent discloses storing the image-forming material and the humidity-controlling material within a package made from a low-moisture permeable film. The low-moisture permeable film can be a plastic film on which is deposited a metal. Other low-moisture permeable films mentioned include fluorinated resins such as polytetrachloroethylene, polytrifluoroethylene, chlorinated rubber, polyvinylidene chloride, a copolymer of polyvinylidene chloride and acrylonitrile, polyethylene, polypropylene, polyesters, and films obtained by depositing a metal such as aluminum and a metal oxide such as silicon oxide.
Unfortunately, when the imaging media are not used right away, especially if a plurality of media are stored for some time in a printing device prior to forming an image, the media may have an opportunity to adjust to ambient humidity and, especially in very dry or very humid climates, the RH of the media may decrease or increase substantially in a short time. Once the imaging media is removed from a package, it does not take very long for the environmental humidity to affect the media. Ambient humidity can soon penetrate the outside surface support on each side of the media causing a change in the moisture content within the media.
After extensive investigation, Applicants have found that humidity affects the mechanical properties of the imaging layers, as compared to the reaction properties of materials during photohardening of the imaging layer. The affect on mechanical properties cause undesirable variations in the degree of rupture of the microcapsules when the media is subjected to pressure during development. Although not wishing to be bound by theory, this may be due to the humidity changing the break strength of the capsules and/or it may be due to the change in pressure applied on the capsules due to the change in the stiffness of the image layer which in turn increases the stress applied to the microcapsules, more likely the latter.
An object of the present invention is, therefore, to provide a self-contained photohardenable imaging assembly that is resistant to the affect of humidity and which will print consistently in response to a means for applying pressure to the assembly.
It would be desirable to obtain an improved media that has no significant change in sensitometric properties with relative humidity, based on speed, Dmax, Dmin, tonal scale, and full color correctness.
It would also be desirable to obtain an improved media that has improved Raw Stock Keeping (RSK), from manufacture to use.
It would be particularly desirable if these objectives could be accomplished without requiring radical changes in conventional imaging chemistry, with respect to the microcapsules and the developer. It would be advantageous if these objectives could be attained in a product that was economical to manufacture and inexpensive for the customer to purchase.
The prior art teaches that the transparent support through which the image is viewed can be formed from any transparent polymeric film, so long as a film is selected that provides good photographic quality when viewing the image and is preferably resistant to yellowing. The transparent support is typically a transparent polyethylene terephthalate (PET) support. Although the prior art use of polyethylene terephthalate (PET) provides a barrier to water vapor transmission to some extent and is transparent, it has now been found that decreasing the water vapor transmission to much lower levels while maintaining the desired thickness and high level of transparency provides unexpected improvement in the performance of the media with respect to pressure development.